SQL Server Median Function - sql-server

Question

CREATE FUNCTION [dbo].[f_Get_Average_Order_Size_Median]
(
#ITEM char(15)
)
RETURNS decimal(21,6)
AS
BEGIN
SELECT #Median = AVG(1.0 * QTYSHP)
FROM
(
SELECT o.QTYSHP, rn = ROW_NUMBER() OVER (ORDER BY o.QTYSHP), c.c
FROM dbo.tbl AS o
WHERE RQDATE >=DATEADD (mm,-6, GETDATE())
AND PRICE != '0'
AND SALESMN != 'WB'
AND item = #ITEM )
+
SELECT o.QTYSHP, rn = ROW_NUMBER() OVER (ORDER BY o.QTYSHP), c.c
FROM tbl
WHERE RQDATE >=DATEADD (mm,-6, GETDATE())
AND PRICE != '0'
AND SALESMN != 'WB'
AND item = #ITEM
CROSS JOIN (SELECT c = COUNT(*)
FROM dbo.tblS) AS c
WHERE RQDATE >=DATEADD (mm,-6, GETDATE())
AND PRICE != '0'
AND SALESMN != 'WB'
AND item = #ITEM
+
(SELECT c = COUNT(*)
FROM dbo.tblS) AS c
WHERE RQDATE >=DATEADD (mm,-6, GETDATE())
AND PRICE != '0'
AND SALESMN != 'WB'
AND item = #ITEM
) AS x
WHERE rn IN ((c + 1)/2, (c + 2)/2);
#Return = #Median
BEGIN
END
RETURN #Return
END TRANSACTION...
Is this the correct median function? Please correct me ..I'm learning

Answer 1

The median is the value that accumulates 50% of the values (the 50% percentile). So I think the simplest way to do it is:
Count the number of records (let's say this count is 'n')
Select the top n / 2 records (if n is even, round it to the next integer value), sorted by the column that holds the value for which you want to calculate the median. Read the biggest (last) value of this column.
I'm not quite familiar with SQL server, but in MySQL I would do it like this:
set #n = (select count(*) from yourTable);
set #med = ceil(#n / 2);
select yourColumn
from (
select yourColumn
from yourTable
order by yourColumn
limit #med
) as a
order by yourColumn desc
limit 1;

Answer 2

For SQL Server 2005+ you could try this solution:
DECLARE #MyTable TABLE
(
ID INT PRIMARY KEY,
Value NUMERIC(9,2)
);
INSERT #MyTable (ID, Value) VALUES (1, 10);
INSERT #MyTable (ID, Value) VALUES (2, 20);
INSERT #MyTable (ID, Value) VALUES (3, 30);
INSERT #MyTable (ID, Value) VALUES (4, 40);
-- Test #1: 4 rows => AVG(20,30)
SELECT AVG(y.Value) AS Median#1
FROM
(
SELECT *,
ROW_NUMBER() OVER(ORDER BY x.ID ASC) AS RowNumASC,
ROW_NUMBER() OVER(ORDER BY x.ID DESC) AS RowNumDESC
FROM #MyTable x
) y
WHERE y.RowNumASC = y.RowNumDESC
OR y.RowNumASC + 1 = y.RowNumDESC
OR y.RowNumASC - 1 = y.RowNumDESC;
-- End of Test #1
-- Test #2: 5 rows => AVG(30)
INSERT #MyTable (ID, Value) VALUES (5, 50);
SELECT AVG(y.Value) AS Median#2
FROM
(
SELECT *,
ROW_NUMBER() OVER(ORDER BY x.ID ASC) AS RowNumASC,
ROW_NUMBER() OVER(ORDER BY x.ID DESC) AS RowNumDESC
FROM #MyTable x
) y
WHERE y.RowNumASC = y.RowNumDESC
OR y.RowNumASC + 1 = y.RowNumDESC
OR y.RowNumASC - 1 = y.RowNumDESC;
-- End of Test #2
Results:
Median#1
---------
25.000000
Median#2
---------
30.000000